The conversation stack provides a convenient means of temporarily pre-empting one topic of conversation for another to an arbitrary recursion depth. When someone wants to pre-empt with a new topic, they push the current topic onto the stack, and the new topic becomes the current topic. When the current topic is exhausted, we pop off the topic on the top of the stack to become the current topic. When the stack is empty, go to bed, 'cause man, it's 4 in the morning and I have class in 5 hours.

Also there's the home stack.